1. Field of the Invention
The present invention relates to polyurethane resins, more particularly, to water-dispersed polyurethane resins which are film formers.
2. Brief Description of the Prior Art
Water-dispersible polyurethanes are known in the art. For example, U.S. Pat. No. 2,968,575 to Mallonee discloses emulsifying NCO-containing prepolymers in a solution of diamine and water with the aid of detergents and under the action of powerful shearing forces. A chain lengthening reaction takes place as water and diamine diffuse into the droplets of the emulsion and react with the isocyanate. The resultant poly(urethane-urea) can then be further processed to form a coating. The process described in U.S. Pat. No. 2,968,575 has the disadvantage associated with it that in order to form the emulsion, a detergent must be used. The detergent usually finds its way into the resultant coating where it can seriously detract from the coating's overall physical and chemical properties. Besides, insufficient shearing force often results in unstable products, and the material can usually not be produced in typical reaction kettles because of the high shearing forces needed.
There have also been suggestions in the prior art to prepare fully reacted polyurethanes in organic solvent with internally contained salt groups which permit the polyurethane to be dispersed in water. For example, U.S. Pat. No. 3,479,310 to Dieterich et al discloses dispersing a fully chain extended, NCO-free polyurethane having internally contained ionic salt groups in water without the aid of detergent.
However, chain extended high molecular weight polyurethanes are very difficult to disperse satisfactorily in water. The resultant dispersions are fairly coarse and they require a high percentage of salt groups for stability. These high percentages of salt groups normally result in coatings which are moisture sensitive. In addition, because the high molecular weight polyurethanes are generally quite high in viscosity, they require extensive thinning with organic solvent before they have a sufficiently low viscosity for dispersion without high shearing forces. The excess solvent must later be removed by steam distillation or the like. Polyurethane dispersions of the present invention, on the other hand, which are prepared by first dispersing a low molecular weight partially reacted NCO-containing prepolymer which contains acid salt groups in an aqueous medium followed by chain extending in the aqueous medium have a finely particulated dispersed phase. By chain extending in aqueous medium, we have found that only a relatively small percentage of salt groups is needed for satisfactory dispersion. In addition, the low molecular weight prepolymer materials have sufficiently low viscosities that they can be dispersed neat at room temperature or in the presence of small amounts of organic solvents. Further chain extension in water does not require additional solvent. It is believed that by making polyurethanes this way, the molecules of polyurethanes are coiled.
The idea of chain extending an NCO prepolymer with internally contained acid salt groups in water with an organic polyamine is generally expected to give gels due to the reaction of polyacids with polyamines. In fact, this method of making crosslinked polyurethanes was generally disclosed in Canadian Pat. No. 837,174, to Witt et al. This reference discloses the preparation of aqueous dispersions of highly crosslinked polyurethanes. The polyurethanes are prepared by dispersing an NCO-containing prepolymer which has internally contained acid salt groups in water. The prepolymer is reacted in water with a polyamine to give a highly crosslinked product. Crosslinking can also occur by using polyvalent counter ions of the ionic groups in the polymer. The process and the products prepared from the Witt et al process differ from the present invention in that they are highly crosslinked rather than ungelled, solvent-soluble products of the present invention. Highly crosslinked products are undesirable because they are not solvent-soluble and will not readily coalesce to form continuous films. For coating or adhesive usage, gel must be avoided. Witt et al do not teach how to make ungelled film-forming polyurethanes.
U.S. Pat. No. 3,868,350 discloses sedimenting aqueous solutions of thermoplastic polyurea powders made by reacting polyurethanes which contain free NCO groups and ionic groups with primary and/or secondary aliphatic diamines and/or dicarboxylic acid-bis-hydrazides at an NH to NCO ratio of from 0.1 to 0.95 in the presence of water. U.S. Pat. No. 3,868,350 acknowledges the difficulties in forming ungelled or uncrosslinked polyurethanes by further reaction of NCO-polymers with chain extenders in the presence of water. The means U.S. Pat. No. 3,868,350 has used to form ungelled products is to react an NCO-polymer of specified salt content with a stoichiometric deficit of a specified chain extender. The final polymer product must have specified urethane, urea and salt group content. Although the resultant products are ungelled, they suffer from numerous shortcomings. The products are sedimenting and not stable dispersions. As such, the sedimented product cannot be used to make coatings without intensive heating (e.g., powder coatings) or strong organic solvents to dissolve the powders. Thus, conventional coating techniques such as spraying, dipping, electrodepositing, electrostatic spraying cannot be employed. Further, since the products of U.S. Pat. No. 3,868,350 are prepared with specified chain extender in a stoichiometric deficit, and since the products must contain a specified urethane, urea and salt group content, products of only a limited range of physical and chemical properties can be produced. Finally, although the patent mentions that the urethane dispersions can be combined with crosslinking agents, there is no disclosed means of how this may be accomplished.
Therefore, from the above, there are numerous shortcomings in the prior art relating to water-dispersed, ungelled polyurethanes. It is surprising that ungelled polyurethane dispersions can be prepared by the present invention. Besides, the polyurethane dispersions of the present invention are also surprisingly superior to those of the prior art, overcoming many of their shortcomings. The polyurethane dispersions of the present invention are ungelled, solvent-soluble materials which are excellent film formers. They can easily be prepared not requiring detergent, high shearing forces, high temperatures or excessive amounts of organic solvent for a satisfactory dispersion. The polyurethane dispersions of the present invention can be prepared extremely fine, making the dispersion stable or non-settling. By this is meant that after the dispersion is prepared, the dispersed phase remains in dispersion and will not form hard sediments. They cannot be filtered by regular means. Besides being non-sedimenting, fine particle size dispersions are advantageous because they have a high surface energy associated with them. This results in a strong driving force for coalescing, and in coatings having surprisingly fast drying times. The polyurethanes of the present invention, although prepared in water, can be deposited as a coating which, when desired, is insensitive to humidity and moisture, which is an unusual combination of properties. Coatings prepared with the polyurethane dispersions of the present invention can be made with outstanding elastomeric properties such as high tensile strength, good ultimate elongation, excellent impact resistance and hardness, in addition to excellent solvent and humidity resistance.